Solid-state batteries based on a metallic Li anode and nonflammable solid electrolytes (SEs) are anticipated to achieve high energy and power densities with absolute safety. In particular, cubic garnet-type Nb-doped LiLaZrO (Nb-LLZO) SEs possess superior ionic conductivity, are feasible to prepare under ambient conditions, have strong thermal stability, and are of low cost. However, the interfacial compatibility with Li metal and Li dendrite hazards still hinder the applications of Nb-LLZO. Herein, a quick and efficient solution was applied to address this issue, generating a nano-LiPO pre-reduction layer from the reaction of HPO with the ion-exchanged passivation layer (LiCO/LiOH) on the surface of Nb-LLZO. A lithiophilic, electrically insulating interlayer is in situ created when the LiPO modified layer interacts with molten Li, successfully preventing the reduction of Nb. The interlayer, which mostly consists of LiP and LiPO, also has a high shear modulus and relatively high Li conductivity, which effectively inhibit the growth of Li dendrites. The Li|LiPO|Nb-LLZO|LiPO|Li symmetric cells stably cycled for over 5000 h at 0.05 mA cm and over 1000 h at a high rate of 0.15 mA cm without any short circuits. The LiFePO and S/C hybrid solid-state batteries using the modified Nb-LLZO electrolyte also demonstrated good electrochemical performances, confirming the practical application of this interfacial engineering in various solid-state battery systems. This work offers an efficient solution to the instability issue between the Nb-LLZO SE and metallic Li anode.